Frames in which left and right front side frames are formed by extrusion from light alloy materials such as aluminum alloys or the like are known, for example, from Japanese Patent Application Laid-Open Publication No. 11-208519 (JP 11-208519 A) and Japanese Patent Application Laid-Open Publication No. 2003-72587 (JP 2003-72587 A).
The front side frames disclosed in JP 11-208519 A and JP 2003-72587 A are hollow members in which the cross-sectional shape of the outer frame part is formed as a regular hexagonal shape, and a plurality of radial ribs that connect the respective vertices formed on the peripheral wall with a central shaft are disposed at equal intervals.
In such front side frames, radial ribs are disposed at equal intervals as a result of the peripheral wall being formed in a regular hexagonal shape. Therefore, when an impact occurs in the front part of the vehicle body, this impact can be uniformly supported by the peripheral wall, radial ribs, and the like. Accordingly, the impact energy that is generated is favorably absorbed by the front side frame.
However, since the abovementioned front side frames have a structure in which the peripheral wall is formed in a regular hexagonal shape, and radial ribs are disposed at equal intervals, the respective vertices are positioned on the circumference of the same circle. Accordingly, the width dimension of the front side frames is increased, and restrictions arise in the formation of a space for accommodating the radiator and the like between the left and right front side frames. Consequently, there is a demand for a technique that can ensure the favorable absorption of the impact energy that is generated, even in front side frames having a short axis and a long axis.